Double channel spectrum analyzer



3,364,426 DOUBLE CHANNEL SPECTRUM ANALYZER Hyman Hurvitz, 326 Woodward Bldg, Washington, D.C. 20005 Filed Jan. 14, 1965, Ser. No. 425,525 8 Claims. (Cl. 32477) ABSTRACT OF THE DISQLOSURE The present invention relates generally to superheterodyne scanning spectrum analyzers and more particularly to scanning spectrum analyzers which operate as corre- Ilators in order to enhance the selectivity obtainable in such analyzers by virture of intermediate filter selectivity alone.

The conventional spectrum analyzer of the scanning superheterodyne type achieves selectivity only in its I.F. filter. As filter selectivity is increased, i.e. filter bandwidth decreased, scanning rate must be correspondingly decreased. For example, to achieve 1 c.p.s. selectivity i.e. if I.F. bandwidth is 1 c.p.s. scan rate must approximate l c.p.s. per second. To scan a 10,000 c.p.s. band then re quires at least 10,000 seconds, about three hours. Additionally, it is nearly impossible to achieve such selectivity even with crystal filters.

In accordance with the present invention greater selec-- tivity of a scanning spectrum analyzer of the superheterodyne type can be achieved than is available in an IF. filter. The result is achieved by two superheterodyne receivers in parallel, which are scanned in synchronism, which may utilize similar I.F. amplifiers but two local oscillators which are different in the two receivers. Values for the IF. filters are selected so that for any common output frequency the outputs of the receivers are cophasal for only one set of local oscillator frequencies, the IF. filters introducing opposite phase shifts for all other local oscillator frequencies. The outputs of the receivers are compared by a correlator circuit, which is phase sensitive and provides maximum output for two signals of the same frequency only if these are cophasal, and zero output for 90 phase. Since some LP. filters introduce a 45 phase shift at each half power point there will be zero output from the system at the half power points of the LF. filters of the receivers. The apparent resolution of the system is thus radically decreased, i.e. effective band width is decreased, with respect to filter band width. Since the filters can be broader than is the case in the prior art if presently available selectivities are to be achieved, the filter can be cheaper and, also, rate of scan can be increased for the wider filters.

It is an object of the invention to provide a scanning spectrum analyzer of the superheterodyne type, which achieves high resolution by means of wide LP. filters, and achieves this by arranging the analyzer to be capable of utilizing correlator techniques.

The above and still further objects, features and ad vantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a block diagram according to the inven tion,

3,364,420 Patented Jam 16, 19685 FIGURES 2A, 2B are filter selectively curves, useful in explaining the invention and FIGURE 3 is a system for tracking two local oscillators with precision over a wide band of frequencies.

In FIGURE 1, S is a source of a band of frequencies F, say in the band. 0-l0 kc. The band of frequencies F is applied to two mixers M M in parallel. The mixer M is supplied with. a local oscillator 0 which scans from 85 kc., when frequency modulated by modulator M in response to the output of sawtooth source ST. Mixer M is supplied with the output of oscillator 0 which scans from 7565 kc., when frequency modulated by modulator M in response to the output of ST. Accordingly, the out put of M is O F. while the output of M is 0 +F. Any frequency scanned through the LP. filters IF and IE then moves in. opposite directions, through the IF. filters.

For example, if the selectivity curves of the filters .lF and IE, are S and S of FIGURES 2A and 2B, respectively, and if an input frequency is 1 kc., this frequency will be converted to points C C at the centers of the bands 8, and S when 0 and 0 are 76, and 74 kc. re-- spectively. If 0 and 0 are simultaneously 75.9 and 74.]. kc. the IF. frequencies will be 74.9 and 75.1 kc. respec tively, i.e. A and A on opposite slopes of the selectivity curves. For simultaneous local oscillator frequencies of 76.1 and 73.9, on the other hand, the IF. frequencies will be 75.1 and 74.9 kc., i.e. B and B These results assume a rather wide I.F., but only to simplify arithmetic. Anal ogous results will occur for any I.F, bandwidth. For the values given, the outputs of the LF. filters will differ in phase by if the selected points are half power points, and the phase difference will only be 0 at C C The outputs of filters IF, and 1P are remixed in mixers M M connected to oscillators O and 0 respectively, which returns the frequencies to their input values, 1000 c.p.s. in the example. The phases of O 0 are random, but this fact is not germane in the present system because the result of double heterodyning is to return the signal to its phase at input F, except for any change which may have occurred in the LP. filters.

The outputs of modulators M M, are multiplied in a four quadrant multiplier X, the output: which is integrated in integrator I. The output of the integrator I is rectified, to provide only one polarity of output, amplified in video amplifier 1.1, and applied to the V plate of oscillograph OS. The output of ST is applied to the H plate. In operation, as the sawtooth provided by ST moves over its assigned set of values, the frequencies of F are converted in. the same sequence and simultaneously to their IlLF. :fre-' quencies. These will be different at all times except at the center of the LP. filter, i.e. at C C As has been shown, A A are derived from the same value of input fre quency but are different values of frequency. However, the second conversion steps, in M M regenerate the same frequencies, equal to the selected input frequency.

.Any phase shift introduced by the IF. filters is not re moved by double conversion, but local oscillator frequencies and. phases are removed, Correlating the 0-l0 kc. outputs of the second mixers M and M provides a maximum value of output for application to plate V only at the center (3 C of the IF. filters where the correlated signals have the same frequencies and phases.

The system of FIGURE 1 requires that 0 and 0 track frequency, although their relative phases may be random. This is a troublesome problem in practice. To avoid the problem recourse is had to the system of FIGURE 3.. In FIGURE 3 the local oscillations 0 are in fact derived by mixing the output of 0 with the output of a crystal controlled fixed frequency oscillator, 0 in balanced mixer M and the required oscillations derived from mixer M by filter man ras Giving consideration to the operatirm oi the system when two adjacent frequencies are present in. the band P1 these .may be present simultaneously in the IF. filters IF and IR. However, while one of the frequencies is present at points C C of the selectivity curves of the LP. filters, the other may be at A A The latter Wlll. then auto-correlate to zero and the one will auto-correlate to a maximum value. For points on the selectivity curve intermediate .A. and the other signal. is (1) attenuated by the filter and (2) at different. phases in the separate filters. These two effects are cumulative. For points outside .A the product of correlation is negative since the total phase separation provided by the LP. filter is greater than 90. Signals are removed by diode D. There is produced, therefore, a display which is sharper for a single frequency than would be available due to LP. selectivity alone and a radical ability to discriminate between adjacent: frequencies While the system. as disclosed utilizes frequencies C and C which are equal, and selectively curves for filters TF and IF; which are the same, this is not essential to the invention. The filter characteristics can be dilfcrent and/or have different center frequencies, providing ap propriate modifications are made in oscillator frequencies, to assure that the same frequencies will pass the filters with zero phase shift at the same time.

While I have described and illustrated one specific em bodiment of my invention, it will be clear that variation of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim;

1.. A spectrum analyzer, comprising a source of. a. wide frequency spectrum, a pair of channels connected in parallcl to said source, one of said channels including in. cascade a. first: mixer, a first narrow band filter and second mixer, a first local oscillator connected iointly to said first and third. mixers, means scanning said first local oscillator over band of frequencies exclusive of said wide frequency spectrum, the frequencies of; said first local oscillator dur in said scanning and of said first narrow band {filter be .ing selected to scan the frequencies of said Wide fre quency spectrum in a. predetermined sequence and in a predetermined direction. through. said first narrow band filter, the other of said channels including in. cascade third mixer, a second narrow band filter, and a. fourth mixer, a. second local, oscillator connected to said third and fourth mixers, means scanning said second oscillator over a. band of frequencies exclusive of said wide fre quency spectrum, the frequencies of said. second local. oscillator during its scanning and of said second. narrow band. filter being selected to scan the frequencies of said wide hand spectrum in said predetermined sequence and in a predetermined. direction. opposite to the first mentinned predctennined direction through. said second nan row band filter, means multiplying the outputs of said 4. second and fourth, mixers occurring within said wide 't'requency spectrum to form a product, means integrating said product to provide an integrated output and means displaying said integrated output as a. function of the fre quencies of said wide frequency spectrum.

2. The combination according to claim 1 wherein said filters are substantially identical.

3. In a spectrum analyzer for analyzing a. broad frequency spectrum, a first channel, a second channel, means applying said broad frequency spectrum to said channels in parallel, each of said channels having an output terminal, said channels including means for providing corresponding frequencies of said broad frequency spectrum. in sequence simultaneously at said output terminals, each of said channels including a source of local oscillations, and means scanning said local oscillators in opposite directions in the separate channels.

4. The combination according to claim. 3 wherein each of said channels includes in. cascade a first heterodyue @Oll' verter, a. narrow band filter, and a second heterodyne con-- verter, said converters of each channel having a common one of said sources of local. oscillations.

5. The combination according to claim 3 wherein is further provided means for correlating the signals at: said output. terminals, and means for displaying a. plot of the output of said means for correlating against frequencies of said broad band spectrum.

6'. The combination according to claim 4, wherein is further provided means for correlating the signals at said output terminals, and means for displaying a plot of the output. of said means for correlating against frequencies of said. broad band spectrum.

7, In a spectrun'r analyzer for analyzing a, broad the quency specmun..n

a first channel,

a second chann l,

means applying said broad frequency spcctrrmi. to said channels in parallel.

each. of said channels being a heterodyne channel and including a scanning local oscillator, means scanning said local. oscillators in respectively opposite senses of scan simultaneously and correlator means connected to the outputs of said channels for providing av display of amplitude versus .frc quency.

a l. The combination according to claim '7 wherein said channels provide always signals of identical frequency to said correlator means.

RI DOILPH IR Ulilll'lt Primary .liiimn'limm Pt WIT dsrfszrmr Examiner, 

